Police in Northern Ireland were dealing with four separate security alerts on Thursday, including an explosion under a car in the capital Belfast, but no one was injured and it was unclear if the incidents were linked to political violence.

Officers were examining an object where a suspected bomb went off late on Wednesday. Part of the road where the incident took place had been closed and residents were evacuated.

"We believe at this stage that a device may have exploded, or partially exploded under a parked car, causing extensive damage to the car," the Police Service of Northern Ireland (PSNI) said in a statement.

Separately, police said they had received reports that a suspected bomb had been left near a railway crossing in Lurgan, outside Belfast, on Wednesday night. The railway line between Belfast and Dublin, the capital of the Republic of Ireland, had been suspended until further notice.

In two other security alerts in Belfast, police closed roads and evacuated residents after suspicious objects were found.

A PSNI spokesman said it was too early to say whether paramilitary groups involved in political violence were behind the incidents, and all lines of inquiry remained open.

Security experts say it is sometimes hard to draw a clear line between organised crime and paramilitary activity in Northern Ireland, with criminals at times taking advantage of the weapons and expertise of former paramilitaries.

Northern Ireland was plagued for decades by violence between Republicans wanting to split from the United Kingdom and become part of the Republic of Ireland, and Unionists opposing that.

The conflict was largely ended by the Good Friday peace agreement of 1998, although some dissidents still reject the power-sharing deal.

Gerry Adams, leader of Republican party Sinn Fein which is represented in Northern Ireland's devolved assembly and government, said, referring to the explosion in Belfast, "Clearly whoever is responsible should desist."

"If you want to keep the union or you want a united Ireland, there's now a peaceful and democratic way so these actions should stop," he said.

The head of Britain's MI5 security service said last month that dissident Republicans continued to pose a threat.

"Whilst there has been great progress in Northern Ireland, dissident Republicans continue to carry out terrorist attacks aimed at the police, prison officers and others," Andrew Parker said. "There were more than 20 such attacks in 2014, most of which, thankfully, were unsuccessful."